Carbon is known to have five unique crystalline structures, including diamond, fullerene (0-D nano graphitic material), carbon nano-tube (1-D nano graphitic material), graphene (2-D nano graphitic material), and graphite (3-D graphitic material).
The carbon nano-tube (CNT) refers to a tubular structure grown with a single wall or multi-wall. Carbon nano-tubes have a diameter on the order of a few nanometers to a few hundred nanometers. Its longitudinal, hollow structure imparts unique mechanical, electrical and chemical properties to the material. CNT is a 1-D (one-dimensional) nano carbon or 1-D nano graphite material.
Bulk natural flake graphite is a 3-D graphitic material with each particle being composed of multiple grains (or graphite single crystals or crystallites) with grain boundaries (amorphous or defect zones) demarcating neighboring graphite single crystals. Each grain is composed of multiple graphene planes oriented parallel to one another. A graphene plane in a graphite crystallite is composed of carbon atoms occupying a two-dimensional, hexagonal lattice. In a given grain or single crystal, the graphene planes are stacked and bonded via van der Waal forces in the crystallographic c-direction (perpendicular to the graphene plane or basal plane). Although all the graphene planes in one grain are parallel to one another, typically the graphene planes in one grain and the graphene planes in an adjacent grain are different in orientation. In other words, the orientations of the various grains in a graphite particle typically differ from one grain to another.
A graphite single crystal (crystallite) is anisotropic with a property measured along a direction in the basal plane (crystallographic a or b direction) being dramatically different than if measured along the crystallographic c-direction (thickness direction). For instance, the thermal conductivity of a graphite single crystal can be up to approximately 1,920 W/mK (theoretical) or 1,800 W/mK (experimental) in the basal plane (crystallographic a- and b-axis directions), but that along the crystallographic c-axis direction is less than 10 W/mK (typically less than 5 W/mK). Consequently, a natural graphite particle composed of multiple grains of different orientations has a property lying between these two extremes. It would be highly desirable in many applications to produce a bulk graphite particle (containing single or multiple grains) having sufficiently large dimensions and having all graphene planes being essentially parallel to one another along one desired direction. For instance, it is highly desirable to have one bulk graphite particle (e.g. a unitary layer entity of multiple graphene planes) with all the graphene planes being substantially parallel to one another) and this unitary layer entity has a sufficiently large length/width for a particular application (e.g. >5 cm2 for use as a heat-spreading sheet on a CPU of a smart phone). It has not been possible to produce this type of large unitary graphene entity from existing natural or synthetic graphite particles.
The constituent graphene planes of a graphite crystallite can be extracted or isolated from a graphite crystallite to form individual graphene sheets of carbon atoms. An isolated, individual graphene sheet is commonly referred to as single-layer graphene. A stack of multiple graphene planes bonded through van der Waals forces in the thickness direction is commonly referred to as a multi-layer graphene, typically having up to 300 layers or graphene planes (<100 nm in thickness), but more typically up to 30 graphene planes (<10 nm in thickness), even more typically up to 20 graphene planes (<7 nm in thickness), and most typically up to 10 graphene planes (commonly referred to as few-layer graphene in scientific community). Single-layer graphene and multi-layer graphene sheets are collectively called “nano graphene platelets” (NGPs). Graphene or NGP is a new class of carbon nano material (a 2-D nano carbon) that is distinct from the 0-D fullerene, the 1-D CNT, and the 3-D graphite.
Our research group pioneered the development of graphene materials and related production processes as early as 2002: (1) B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006), application submitted in October 2012; (2) B. Z. Jang, et al. “Process for Producing Nano-scaled Graphene Plates,” U.S. patent application Ser. No. 10/858,814 (Jun. 3, 2004); and (3) B. Z. Jang, A. Zhamu, and J. Guo, “Process for Producing Nano-scaled Platelets and Nanocomposites,” U.S. patent application Ser. No. 11/509,424 (Aug. 25, 2006).
NGPs are typically obtained by intercalating natural graphite particles with a strong acid and/or oxidizing agent to obtain a graphite intercalation compound (GIC) or graphite oxide (GO), as illustrated in FIG. 1(a) (process flow chart) and FIG. 1(b) (schematic drawing). This is most often accomplished by immersing natural graphite powder (20 in FIGS. 1(a) and 100 in FIG. 1(b)) in a mixture of sulfuric acid, nitric acid (an oxidizing agent), and another oxidizing agent (e.g. potassium permanganate or sodium chlorate). The resulting GIC (22 or 102) is actually some type of graphite oxide (GO) particles. This GIC is then repeatedly washed and rinsed in water to remove excess acids, resulting in a graphite oxide suspension or dispersion, which contains discrete and visually discernible graphite oxide particles dispersed in water. There are two processing routes to follow after this rinsing step:
Route 1 involves removing water from the suspension to obtain “expandable graphite,” which is essentially a mass of dried GIC or dried graphite oxide particles. Upon exposure of expandable graphite to a temperature in the range of typically 800-1,050° C. for approximately 30 seconds to 2 minutes, the GIC undergoes a rapid expansion by a factor of 30-300 to form “graphite worms” (24 or 104), which are each a collection of exfoliated, but largely un-separated or still interconnected graphite flakes. A SEM image of graphite worms is presented in FIG. 2(a).
In Route 1A, these graphite worms (exfoliated graphite or “networks of interconnected/non-separated graphite flakes”) can be re-compressed to obtain flexible graphite sheets or foils (26 or 106) that typically have a thickness in the range of 0.125 mm (125 μm)-0.5 mm (500 μm). One may choose to use a low-intensity air mill or shearing machine to simply break up the graphite worms for the purpose of producing the so-called “expanded graphite flakes” (108) which contain mostly graphite flakes or platelets thicker than 100 nm (hence, not a nano material by definition).
Exfoliated graphite worms, expanded graphite flakes, and the recompressed mass of graphite worms (commonly referred to as flexible graphite sheet or flexible graphite foil) are all 3-D graphitic materials that are fundamentally different and patently distinct from either the 1-D nano carbon material (CNT) or the 2-D nano carbon material (graphene).
As disclosed by M. Smalc, et al, U.S. Pat. No. 7,292,441 (Nov. 6, 2007) and No. 6,982,874 (Jun. 3, 2006), and J. W. Tzeng, U.S. Pat. No. 6,482,520 (Nov. 19, 2002), these flexible graphite (FG) foils can be used as a heat spreader material, but exhibiting a maximum in-plane thermal conductivity of typically less than 500 W/mK (more typically <300 W/mK) and in-plane electrical conductivity no greater than 1,500 S/cm. These low conductivity values are a direct result of the many defects, wrinkled or folded graphite flakes, interruptions or gaps between graphite flakes, and non-parallel flakes (e.g. SEM image in FIG. 2(b)). Many flakes are inclined with respect to one another at a very large angle (e.g. mis-orientation of 20-40 degrees).
In Route 1B, the exfoliated graphite is subjected to high-intensity mechanical shearing (e.g. using an ultrasonicator, high-shear mixer, high-intensity air jet mill, or high-energy ball mill) to form separated single-layer and multi-layer graphene sheets (collectively called NGPs, 33 or 112), as disclosed in our U.S. application Ser. No. 10/858,814. Single-layer graphene can be as thin as 0.34 nm, while multi-layer graphene can have a thickness up to 100 nm. In the present application, the thickness of multi-layer NGPs is typically less than 20 nm.
Route 2 entails ultrasonicating the graphite oxide suspension for the purpose of separating/isolating individual graphene oxide sheets from graphite oxide particles. This is based on the notion that the inter-graphene plane separation has been increased from 0.335 nm in natural graphite to 0.6-1.1 nm in highly oxidized graphite oxide, significantly weakening the van der Waals forces that hold neighboring planes together. Ultrasonic power can be sufficient to further separate graphene plane sheets to form separated, isolated, or discrete graphene oxide (GO) sheets. These graphene oxide sheets can then be chemically or thermally reduced to obtain “reduced graphene oxides” (RGO) typically having an oxygen content of 0.01%-10% by weight, more typically 0.01%-5% by weight.
For the purpose of defining the claims of the instant application, NGPs include single-layer and multi-layer graphene or reduced graphene oxide with an oxygen content of 0-10% by weight, more typically 0-5% by weight, and preferably 0-2% weight. Pristine graphene has essentially 0% oxygen. Graphene oxide (including RGO) can have 0.01%-46% by weight of oxygen. The graphene oxide gel, to be described in detail later, typically contains 20-46% by weight oxygen. The graphene oxide gel-derived unitary graphene layer or graphene single crystal of the present invention typically has an oxygen content of 0.01% to 5% by weight, more typically less than 2% by weight.
It may be noted that flexible graphite foils (obtained by re-compressing exfoliated graphite worms) for electronic device thermal management applications (e.g. as a heat spreader) have the following major deficiencies:                (1) As indicated earlier, flexible graphite (FG) foils exhibit a relatively low thermal conductivity, typically <500 W/mK and more typically <300 W/mK.        (2) Flexible graphite foils are also of low strength and poor structural integrity. The high tendency for flexible graphite foils to get torn apart makes them difficult to handle in the process of integrating them in a microelectronic device.        (3) Another very subtle, largely ignored or overlooked, but critically important feature of FG foils is their high tendency to get flaky with graphite flakes easily coming off from FG sheet surfaces and emitting out to other parts of a microelectronic device. These highly electrically conducting flakes (typically 1-500 μm in lateral dimensions and >100 nm in thickness) can cause internal shorting and failure of electronic devices.        (4) For this reason, it is necessary to apply a protective resin coating onto a surface or on both surfaces of a flexible graphite foil in order to prevent graphite flakes from being released. This resin coating is typically not a thermally or electrically conductive material that is often an undesirable feature in a situation where high conductivity is required. In other situations where electrical insulation or isolation is required, this resin layer can present some issues (e.g. mis-match in coefficients of thermal expansion and elastic constants between the FG layer and the resin coating, resulting in delamination or peeling-off after some number of thermal cycles).        
Other sheet-like graphitic materials that can be used as a heat spreader or thermal interface material include carbon nano-tube (CNT) paper (e.g. Bucky paper), carbon fiber mat (e.g. carbon nano-fiber or CNF mat), and carbon paper (e.g. made of short carbon fibers). These graphitic sheets also suffer from similar shortcomings as FG foils. For instance, although individual CNTs or CNFs alone can exhibit a high thermal conductivity (1,500-3000 W/mK), the resulting CNT or CNF paper or mat typically exhibit an in-plane thermal conductivity less than 100 W/mK and often less than 10 W/mK, likely due to the few and poor contacts between individual CNT or CNF filaments, providing insufficient cross-sections for electron flow or even impeding electron flow. Further, the contact between a sheet-like graphitic layer and a heat source is usually poor due to limited contact surfaces between such a graphitic layer (e.g. CNT paper) and a rigid device component (e.g. a CPU in a mobile phone). This results in an ineffective heat transfer between the heat source and the graphitic layer.
Similarly, the NGPs (including discrete platelets of pristine graphene and GRO), when packed into a film or paper sheet (34 or 114) of non-woven aggregates, typically do not exhibit a high thermal conductivity. The thermal conductivity is found to be higher than 1,000 W/mK only when the film or paper is cast and pressed into a sheet having a thickness lower than 10 μm, and higher than 1,500 W/mK only when the film or paper is cast and greatly pressed into a sheet having a thickness lower than 1 μm. This is reported in our earlier U.S. patent application Ser. No. 11/784,606 (Apr. 9, 2007). However, ultra-thin film or paper sheets (<10 μm) are difficult to produce in mass quantities, and difficult to handle when one tries to incorporate these thin films as a heat spreader material during the manufacturing of microelectronic devices. Further, thickness dependence of thermal conductivity (not being able to achieve a high thermal conductivity at a wide range of film thicknesses) is not a desirable feature.
In general, paper made from platelets of graphene, graphene oxide, and RGO (e.g. those paper sheets prepared by vacuum-assisted filtration process) exhibit many defects, wrinkled or folded graphene sheets, interruptions or gaps between platelets, and non-parallel platelets (e.g. SEM image in FIG. 3(b)), leading to poor thermal conductivity and electric conductivity. These papers or aggregates of discrete NGP, GO or RGO platelets also have a tendency to get flaky, emitting conductive particles into air.
Our earlier application (U.S. application Ser. No. 11/784,606) further disclosed a mat, film, or paper of NGPs infiltrated with a metal, glass, ceramic, resin, and CVD graphite matrix material. Later on, Haddon, et al (US Pub. No. 2010/0140792, Jun. 10, 2010) also reported NGP thin film and NGP-polymer composites for thermal management applications. The processes used by Haddon et al to produce NGPs are identical to those disclosed much earlier by us (Jang, et al. U.S. patent application Ser. No. 10/858,814 (Jun. 3, 2004)). The NGP-polymer composites, as an intended thermal interface material, have very low thermal conductivity, typically <<2 W/mK. The NGP films of Haddon, et al are essentially non-woven aggregates of discrete graphene platelets, identical to those of our earlier invention (U.S. application Ser. No. 11/784,606). Again, these aggregates have a great tendency to have graphite particles flaking and separated from the film surface, creating internal shorting problem for the electronic device containing these aggregates. They also exhibit low thermal conductivity unless made into thin films (10 nm-300 nm, as reported by Haddon, et al) which are very difficult to handle in a real device manufacturing environment. Balandin, et al (US Pub. No. 2010/0085713, Apr. 8, 2010) also disclosed a graphene layer produced by CVD deposition or diamond conversion for heat spreader application. More recently, Kim, et al (N. P. Kim and J. P. Huang, “Graphene Nanoplatelet Metal Matrix,” US Pub. No. 2011/0108978, May 10, 2011) reported metal matrix infiltrated NGPs. However, metal matrix material is too heavy and the resulting metal matrix composite does not exhibit a high thermal conductivity.
Another prior art material for thermal management application is the pyrolitic graphite film. The lower portion of FIG. 1(a) illustrates a typical process for producing prior art pyrolitic graphitic films or sheets from a polymer. The process begins with carbonizing a polymer film 46 at a carbonization temperature of 500-1,000° C. for 2-10 hours to obtain a carbonized material 48, which is followed by a graphitization treatment at 2,500-3,200° C. for 5-24 hours to form a graphitic film 50. This is a slow, tedious, and energy-intensive process. Furthermore, carbonization of certain polymers (e.g. polyacrylonitrile) involves the emission of toxic species.
Another type of pyrolytic graphite is produced by high temperature decomposition of hydrocarbon gases in vacuum followed by deposition of the carbon atoms to a substrate surface. This is essentially a chemical vapor deposition (CVD) process. In particular, highly oriented pyrolitic graphite (HOPG) is the material produced by the application of uniaxial pressure on deposited pyrocarbon or pyrolytic graphite at very high temperatures (typically 3,000-3,300° C.). This entails a thermo-mechanical treatment of combined mechanical compression and ultra-high temperature for an extended period of time in a protective atmosphere; a very expensive, energy-intensive, and technically challenging process. The process requires high vacuum and ultra-high temperature equipment that is not only very expensive to make but also very expensive and difficult to maintain. Even with such extreme processing conditions, the resulting PG (including HOPG) still possesses many defects, grain boundaries, and mis-orientations (neighboring graphene planes not parallel to each other), resulting in less-than-satisfactory in-plane properties. Typically, the best prepared HOPG sheet or block remains far from being a graphite single crystal; instead, it typically still contains many grains or single crystals and a vast amount of grain boundaries and defects. In general, the PG or HOPG is free from any element than carbon.
Similarly, the most recently reported graphene thin film (<2 nm) prepared by catalytic CM of hydrocarbon gas C2H4) on Ni or Cu surface is not a single-grain crystal, but a poly-crystalline structure with many grain boundaries and defects [e.g., Piran R. Kidambi, et al., “The Parameter Space of Graphene Chemical Vapor Deposition on Polycrystalline Cu,” The Journal of Physical Chemistry C2012 116 (42), 22492-22501]. With Ni or Cu being the catalyst, carbon atoms obtained via decomposition of hydrocarbon gas molecules at 800-1,000° C. are deposited onto Ni or Cu foil surface to form a sheet of single-layer or few-layer graphene that is poly-crystalline. The grains are typically much smaller than 100 μm in size and, more typically, smaller than 10 μm in size. These graphene thin films, being optically transparent and electrically conducting, are intended for touch screen (to replace indium-tin oxide or ITO glass) or semiconductor (to replace silicon. Si) applications. However, these polycrystalline graphene films are not sufficiently thermally conducting (too many grains or too much grain boundaries, and all grains being oriented in different directions) and not sufficiently thick for use as a heat spreader in an electronic device.
Thus, it is an object of the present invention to provide a graphene oxide (GO) gel-derived unitary or monolithic film, which exhibits a thermal conductivity comparable to or greater than that of the PG, HOPG, or CVD graphene film.
It is a specific object of the present invention to provide a GO gel-derived unitary or monolithic entity, which has the following characteristics (separately or in combination): (1) This unitary entity is an integrated graphene object that is either a graphene single crystal (single grain only) or a poly-crystal (multiple grains but having incomplete grain boundaries) with all graphene planes in all grains being essentially oriented parallel to one another (the crystallographic c-axis of all grains being essentially parallel to one another). (2) This integrated graphene entity is not an aggregate or stack of multiple discrete graphite flakes or discrete platelets of graphene or GO, and does not contain any discernible or discrete flake/platelet. (3) This integrated graphene entity is not made by gluing or bonding discrete flakes/platelets together with a binder, linker, or adhesive. Instead, GO molecules in the GO gel are merged, mainly edge-to-edge through joining or forming of covalent bonds with one another, into an integrated graphene entity, without using any externally added linker or binder molecules or polymers. (4) This unitary or monolithic graphene entity (a single crystal or poly-crystal with all graphene planes having crystallographic c-axis essentially parallel to each other) is derived from a GO gel, which is in turn obtained from heavy oxidation of natural graphite or artificial graphite particles originally having multiple graphite crystallites. Prior to being chemically oxidized to become GO gel, these starting graphite crystallites have an initial length (La in the crystallographic a-axis direction), initial width (Lb in the b-axis direction), and thickness (Lc in the c-axis direction). This unitary graphene entity typically has a length or width significantly greater than the La and Lb of the original crystallites.
The present invention also provides a method or process for producing such a GO gel-derived unitary or monolithic graphene entity, or a graphene single crystal (including a graphene poly-crystal with an incomplete grain boundary). The process begins with preparation of a mass of GO gel preferably in a layer form (preferably less than 10 mm in thickness, more preferably less than 1 mm, and most preferably less than 500 μm in thickness prior to drying). The liquid component of this GO gel is then partially or totally removed and, concurrently or sequentially, this GO layer is thermally converted to an integrated graphene film obtained by heat-treating graphene oxide gel to chemically merge individual graphene oxide molecules primarily in an edge-to-edge manner.
Another object of the present invention is to provide a cost-effective process of producing GO-derived graphene monolith that exhibits a combination of exceptional thermal conductivity, electrical conductivity, mechanical strength, surface hardness, and scratch resistance unmatched by any thin-film graphitic material of comparable thickness range.
In particular, the present invention provides a process for producing a unitary or monolithic graphene layer or graphene single crystal from a GO gel. This process does not involve or require an ultrahigh temperature as is absolutely required of the processes for producing pyrolytic graphite (including HOPG) from either carbonized polymers (e.g. polyimide) or using the CVD deposition. The presently invented process is simple, less energy-intensive, and highly scalable.
This thermally and electrically conductive graphene monolith can be used for thermal management applications (e.g. for use as a heat spreader) in a microelectronic device, such as a mobile phone (including a smart phone), a notebook computer, a tablet, an e-book, a telecommunication device, and any hand-held computing device or portable microelectronic device.
It is another object of the present invention to provide a GO-derived unitary graphene entity that exhibits a combination of exceptional thermal conductivity, electrical conductivity, mechanical strength, surface smoothness, surface hardness, and scratch resistance unmatched by any thin-film material of comparable thickness range.
It is a specific object of the present invention to provide a highly conductive graphene monolith thin-film sheet that meets the following technical requirements (a) in-plane thermal conductivity greater than 600 W/mK (preferably greater than 1,000 W/mK, and further preferably greater than 1,700 W/mK); (b) in-plane electrical conductivity greater than 2,000 S/cm (preferably >3,000 S/cm, more preferably >5,000 S/cm, and most desirably >10,000 S/cm); (c) Rockwell surface hardness value >60 (preferably >80); and/or (d) a tensile strength greater than 10 MPa (preferably >40 MPa, more preferably >60 MPa, and most preferably >100 MPa).